Finely dice the bacon. Clean, wash and chop the spring onions. Peel, wash and quarter the potatoes. Heat oil in a pot. Leave out the bacon, add the spring onions, toss briefly, remove everything.
Add 1 litre of water and potatoes. Season with salt, pepper (and bay leaf). Bring to the boil, cover and cook for about 20 minutes. Coarsely mash potatoes in the liquid. Add cream and bacon mixture, bring to the boil briefly, season to taste again.
Cut the trout fillets into small pieces and warm them carefully in the soup.